Downy Mildew


This is a disease that can affect annuals, perennials, houseplants, fruit and vegetables.

Downy mildew infection can be sometimes be mistaken for powdery mildew.

Unlike powdery mildews, downy mildews are not true fungi, they are more closely related to algae.

Downy mildews are specialised pathogens, requiring a living host on which to grow, and are genus specific, e.g Onion downy mildew.

This means a species attacking one genus will not affect another genus.

In most cases downy mildew will not kill a plant, but it can soon lead to loss of yield or quality.


These are seen as off-white downy fungal growths that appear on the underside of the leaves of affected plants.

Depending on the mildew species,this results in white, grey or purplish-brown blotches appearing on the upper surface of the leaves.

Sometimes the pathogen may systemically spread through the plant, causing the plant to become discoloured,distorted and stunted.

Affected leaf tissue may eventually turn brown and die, and on occasions this can kill the plant.


Remove the affected leaves as soon as possible.

The problem is caused by high humidity around the plants so improving the ventilation in the growing area can prevent the problem.

Leaf wetness and humidity in the growing area should be avoided.

Similarly, be careful when watering and don’t water overhead, and ensure the compost/soil is free draining.

There are no protective or curative fungicides available to the amateur grower.

Fungicides used to control potato blight may have some affect on downy mildews.

Top of the Page